1. Field of the Invention
In general, the invention pertains to a method for controlling the pyrolysis of a complex waste stream of plastics to convert the stream into useful high value monomers and useful chemicals, thereby minimizing disposal requirements for non-biodegradable materials and conserving non-renewable resources. The method uses fast pyrolysis for sequentially converting a plastic waste feed stream having a mixed polymeric composition into high value monomer products by:
using molecular beam spectrometry (MBMS) techniques to characterize the polymeric components of the feed stream and determine process parameter conditions; and differentially heating the feed stream according to a heat rate program using predetermined MBMS data to sequentially obtain optimum quantities of high value monomer products.
If, prior to differential heating, the feed stream catalytically treated to affect thermal reaction pathways, more char is made faster, and the char is a useful chemical in resins and other applications.
The invention achieves heretofore unattained control of a pyrolysis process, as applied to mixed polymeric waste, through greater discovery of the mechanisms of polymer pyrolysis and its utilization, as provided through the use of molecular beam mass spectrometry. Pyrolysis mass spectrometry is used to characterize the major polymers found in the waste mixture, and the MBMS techniques are used on large samples in a manner such that heterogeneous polymeric materials can be characterized in very short time (real time analysis). After characterization, in accordance with the method of the invention, when given a specific waste stream polymer mixture, that mixture is subjected to a controlled heating rate program for maximizing the isolation of desired monomer products, due to the fact that the kinetics of the depolymerization of these polymers have been determined as well as the effects of catalytic (acid/basic) pretreatment.
2. Description of the Prior Art
U.S. Pat. No. 3,546,251 pertains to the recovery of epsilon-caprolactone in good yield from oligomers or polyesters by heating at 210.degree.-320.degree. C. with 0.5 to 5 parts wt. of catalyst (per 100 parts wt. starting material) chosen from KOH, NaOH, alkali earth metal hydroxides, the salts of alkali metals, e.g. Co and Mn and the chlorides and oxides of divalent metals.
U.S. Pat. No. 3,974,206 to Tatsumi et al. discloses a process for obtaining a polymerizable monomer by: contacting a waste of thermoplastic resin with a fluid heat transfer medium; cooling the resulting decomposed product; and subjecting it to distillation. This patent uses not only the molten mixed metal as an inorganic heat transfer medium (mixtures or alloys of zinc, bismuth, tin, antimony, and lead, which are molten at very low temperatures) alone or in the presence of added inorganic salts, such as sodium chloride, etc., by an additional organic heat transfer medium, so that the plastic waste does not just float on the molten metal, and thereby not enjoy the correct temperatures for thermal decomposition. The molten organic medium is a thermoplastic resin, and examples of other waste resins such as atactic polypropylene, other polyolefins, or tar pitch. The added thermoplastic is also partially thermally decomposed into products that end up together with the desired monomers, and therefore, distillation and other procedures have to be used to obtain the purified monomer.
However, Tatsumi et al. provides no means for identifying catalyst and temperature conditions that permit decomposition of a given polymer in the presence of others, without substantial decomposition of the other polymers, in order to make it easier to purify the monomer from the easier to decompose plastic or other high-value chemicals from this polymer.
U.S. Pat. No. 3,901,951 to Nishizaki pertains to a method of treating waste plastics in order to recover useful components derived from at least one monomer selected from aliphatic and aromatic unsaturated hydrocarbons comprising: melting the waste plastic, bringing the melt into contact with a particulate solid heat medium in a fluidized state maintained at a temperature of between 350.degree. to 550.degree. C. to cause pyrolysis of the melt, and collecting and condensing the resultant gaseous product to recover a mixture of liquid hydrocarbons; however, even though one useful monomer is cited, the examples produce mixtures of components, all of which must be collected together and subsequently subjected to purification. No procedure is evidenced or taught for affecting fractionation in the pyrolysis itself by virtue of the catalysts and correct temperature choice.
U.S. Pat. No. 3,494,958 to Mannsfeld et al. is directed to a process for thermal decomposition of polymers such as polymethyl methacrylate using the fluidized bed approach, comprising: taking finely divided polymers of grain size less than 5 mm and windsifting and pyrolysing said polymer grains at a temperature which is at least 100.degree. C. over the depolymerization temperature to produce monomeric products; however, this is a conventional process that exemplifies the utility of thermal processing in general for recovery of specific acrylates. The process of this patent does not acknowledge the need of taking the recovery a step further in the case of more complex mixtures of products, let alone provide a means for doing so.
U.S. Pat. Nos. 4,108,730 and 4,175,211 to Chen et al. relate respectively to treating rubber wastes and plastic wastes by size reducing the wastes, removing metals therefrom, and slurrying the wastes in a petroleum - derived stream heated to 500.degree.-700.degree. F. to dissolve the polymers. The slurry is then fed into a zeolite catalytic cracker operating at 850.degree. F. and up to 3 atmospheres to yield a liquid product, which is a gasoline-type of product (or a mixture of many hydrocarbon products).
While the Chen et al. references exemplify catalytic conversion, it is to a mixture of hydrocarbons boiling in the gasoline range, and not to make specific useful compound(s), which can be isolated by virtue of temperature programming and catalytic conditions.
U.S. Pat. No. 3,829,558 to Banks et al. is directed to a method of disposing of plastic waste polluting the environment comprising: passing the plastic to a reactor, heating the plastic in the presence of a gas to at least the decomposition temperature of the plastic, and recovering decomposition products therefrom. The gas used in the process is a heated inert carrier gas (as the source of heat).
The method of this patent pyrolyses the mixtures of PVC, polystyrene, polyolefins (in equal proportions) at over 600.degree. C., with steam heated at about 1300.degree. F., and makes over 25 products, which were analyzed for, including in the order of decreasing importance, HCl, the main products, butenes, butane, styrene, pentenes, ethylene, ethane, pentane and benzene, among others.
In Banks, no attempt is made to try to direct the reactions despite the fact that some thermodynamic and kinetic data are obtained.
U.S. Pat. No. 3,996,022 to Larsen discloses a process for converting waste solid rubber scrap from vehicle tires into useful liquid, solid and gaseous chemicals comprising: heating at atmospheric pressure a molten acidic halide Lewis salt or mixtures thereof to a temperature from about 300.degree. C. to the respective boiling point of said salt in order to convert the same into a molten state; introducing into said heated molten salt solid waste rubber material for a predetermined time; removing from above the surface of said molten salt the resulting distilled gaseous and liquid products; and removing from the surface of said molten salt at least a portion of the resulting carbonaceous residue formed thereon together with at least a portion of said molten salt to separating means from which is recovered as a solid product, the solid carbonaceous material.
In the Larsen reference, the remainder from the liquid and gaseous fuel products is char. Moreover, these products are fuels and not specific chemicals.
Table 1 provides a summarization of the prior art processes for the thermal decomposition of polymers. TBL3 TABLE 1 Thermal decomposition of polymers (adapted from Buekens) Process developed Reactor type & heating Reaction Plant capacity, by method temperature, .degree.C. tons/day Feedstock Products References a) Union Carbide Extruder, followed by 420-600 0.035-0.07 PE, PP, PS, PVC, PETP, Waxed [1] annular pyrol, tube, PA, mixes electrically heated b) Japan Steel Works Extruder [2] c) Japan Gasoline Co. Tubular reactor, externally Dissolved or suspended in Heavy-oil [2] heated recycle-oil d) Prof. Tsutsumi Tubular reactor, 500-650 1 PS-foam [3] superheated steam as a heat carrier e) Nichimen* Catalytic fixed by reactor Mixed plast, no char- [2] forming polymers f) Toyo Engineering Fluidized by catalytic 0.5 Mixed plast., no char- [2,4] Corp. reactor forming polymers g) Mitsui Shipbuilding & Stirred tank reactor, 420-455 24-30 Low mol. w. polymers (PE, Fuel-oil [5,4] Engineering Co. polymer bath APP0 h) Mitsui Petrochemical Industries Co. (Chiba Works) i) Mitsubishi Heavy Ind. Tank reactor with 400-500 0.7/2.4 Polyolefins Naphtha kerosene [6,4] (Mihara Works) circulation pump and fuel-oil reflux cooling j) Kawasaki Heavy Ind. Polymer bath, formed by 400-450 5 Mixed plast. PE + PS Gas-oil HCL [2,4] (Kakogawa Works) PE and PS content 55% k) Ruhrchemie AG, Stirred tank reactor, salt 380-450 1.2 PE Oil, wax [7] Oberhausen bath l) Japan Gasoline Co. Fluidized bed 450 0.2 PS-waste [8] FIG. 16 m) Prof. Sinn, Univ. of Fluidized bed 640-840 Laboratory scale PE, PS, PVC tyre rubber Aromatic hydro- See [8] Hamburg Prog. Kaminsky Molten salt bath 600-800 Laboratory scale carbons & fuel oil n) Sanyo Electric Co. Tubular reactor with a 260 (PVC), followed 0.3 (pilot) Foam PS, mixed plast. Monomer [8,3,4] screw for carbon removal, by 500-550 3 (Gifu) (select, collect.) asphaltFuel-oil dielectric heating 5 (Kusatsu) 6% S HCL o) Sumitomo Shipbuild. & Fluidized bed, partial 450-470 3-5 Mixed plastics incl. PVC Heavy oil [8,4] Machinery Co. oxidation 600 (28) HCL HCL (Hiratsuka Lab.) p) Government Industrial Fluidized bed, partial 400-510 Bed diameter: 3.5/ PS-chips Monomer and dimer [9] Research Institute oxidation 550 15/30/50 & 120 cm Gasific. prod. q) Nippon Zeon, Japan Fluidized bed, partial 350-600 24 pre-commercial Sheared tyres Gas, oil, char [10] Gasoline Co. oxidation (400-500 mostly) plant (Tokuyama) r) Kobe Steel Externally heated, rotary 600-800 5 (pilot) Crushed tyres Gas, oil, char [8,11] kiln s) Bureau of Mines/ Electrically heated retort 500/900 Laboratory scale Tyre cuttings Gas, oil, char [12] Firestone t) Hydrocarbon Research Autoclave 350-450 Tyres [8] Inc. u) Zeplichal Conveyor band, vacuum Tyres [8] v) Herbold, W. Germany Tyres [13] References [1] J. E. Potts, Reclamation of plastic waste by pyrolysis, Am. Chem. Soc., Div. Water, Air, and Waste Chemistry, Chicago, p. 229, 13-18 September (1970). [2] M. Endo, Techniques for pyrolysing plastics waste, Japan Plastics Q. 29, October (1973). [3] S. Tsutsumi, Thermal and steam cracking of waste plastics, Conference Papers of the First International Conference, Conversion of refuse to energy, Montreux (Switzerland), 4 November (1975) p. 567. [4] Staff Writers, Trends in development of plastic waste disposal techniques, Cem. Econ. Engng. Rev. 4, (10), 29 (1972). [5] Y. Kitaoka and H. Sueyoshi, Conversion of waste polymer to fuel oil, Conference Papers of the First International Conference, Conversion of refuse to energy, Montreux (Switzerland), 4 November (1975) p. 555. [6] K. Matsumoto, S. Kurisu and T. Oyamoto, Development of process of fue recovery by thermal decomposition of waste plastics, Conference Papers of the First International Conference, Conversion of refuse to energy, Montreux (Switzerland) 4 November (1975) p. 538. [7] S. Speth, Aufarbeitung von PolyathylenRuckstanden zu Niedermolekulare Destillaten, Chemie Ing. Tech. 45, (8), 526 (1973). [8] W. Kaminsky, J. Menzel and H. Sinn, Recycling of plastics, Conservation and Recycling, 1, (1), 91 (1976). [9] S. Mitsui, H. Nishizaki and K. Yoshida, Communication at ACHEMA, Frankfurt (1976). [10] Y. Saeki and G. Suzuki, Fluidized thermal cracking process for waste tire, Rubber Age, February (1976). [11] A. Takamura, K. Inque and T. Sakai, Resources recovery by pyrolysis of waste tyres, Conference Papers of the First International Conference, Conversion of refuse to energy, Montreux (Switzerland), 4 November (1975) p. 532. [12] J. A. Beckman, D. J. Bennett, A. G. Altenau and J. R. Laman, Yields and analyses of the products from the destructive distillation of scrap tyres, Conference Papers of the First International Conference, Conversio of refuse to energy, Montreux (Switzerland), 4 November (1975) p. 195. [13] H. W. Schnecko, Zur Pyrolyse von Altreifen, Chem. Ing. Tech. 48 (5), 443 (1976).